Method to be performed by stylus and sensor controller, stylus, and sensor controller

ABSTRACT

A method to be performed by a sensor controller that periodically transmits an uplink signal, and by a stylus that, upon receiving the uplink signal, determines a transmission and reception schedule for a downlink signal and the uplink signal on the basis of the reception timing of the uplink signal. The method includes executing, by the stylus, an uplink signal receiving operation for a first predetermined period of time according to the transmission and reception schedule. The method includes, responsive to the uplink signal not received through the receiving operation and expiration of the first predetermined period of time, continuing, by the stylus, the uplink signal receiving operation for a second predetermined period of time, instead of transmitting the downlink signal according to the transmission and reception schedule.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method to be performed by a stylusand a sensor controller, the stylus, and the sensor controller.

2. Description of the Related Art

Electronic devices that can handle handwriting inputs using styluseshave been known. An electronic device of this type typically includes atouch sensor that is disposed in a touch surface, and a sensorcontroller that is connected to the touch sensor. The sensor controlleris an integrated circuit having a function of performing bidirectionalcommunication with a stylus via the touch sensor. The sensor controlleris configured to detect the position of a stylus on the touch surface bytransmitting an uplink signal via the touch sensor and receiving, viathe touch sensor, a downlink signal transmitted from the stylus inresponse to the uplink signal. The uplink signal is used to notify thestylus of transmission and reception timings of downlink and uplinksignals, and to transmit a command for specifying data to be transmittedfrom the stylus. The uplink signal is transmitted by the sensorcontroller in a fixed cycle.

PCT Patent Publication No. WO2016/129194 discloses an example of astylus that transmits a downlink signal on the basis of an uplink signalreception timing. The stylus of this example initially operates in adiscovery state of performing an uplink signal receiving operation in afixed cycle. When an uplink signal is received as a result of thereceiving operation, the stylus is shifted to an operation state ofperforming bidirectional communication with a sensor controller. Thestylus in the operation state executes a downlink signal transmittingoperation and an uplink signal receiving operation according to atransmission and reception schedule which is determined on the basis ofthe uplink signal reception timing. When reception of an uplink signalaccording to the transmission and reception schedule fails, the stylusis returned to a discovery state to execute an uplink signal receivingoperation.

The sensor controller disclosed in PCT Patent Publication No.WO2016/129194 is configured to, upon receiving a downlink signal fromthe stylus, establish pairing with the stylus, and to cancel the pairingwith the stylus when receiving no downlink signal from the stylus for afixed period of time or longer. Therefore, when the stylus returns tothe discovery state, the pairing is canceled by the sensor controllerafter a while.

A stylus that is located at a position near a touch surface where thestylus can normally receive an uplink signal may, in some cases, fail toreceive an uplink signal. In such cases, transmission of downlinksignals from the stylus is suspended. Thus, the sensor controllerbecomes unable to detect the position of the stylus, and further,pairing is also canceled after a while. To users, this occurrencegenerates a situation where the user's writing operation is suddenlydisabled. A need exists for a remedial measure against such situation.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to providing a method,which is to be executed by a stylus and a sensor controller, foravoiding occurrence of a situation where writing that is being executedis suddenly disabled, and the related stylus and the related sensorcontroller.

A method according to one aspect of the present invention is performedby a sensor controller that periodically transmits an uplink signal, andby a stylus that, upon receiving the uplink signal, determines atransmission and reception schedule for a downlink signal and the uplinksignal on the basis of the reception timing of the uplink signal. Themethod includes: by the stylus, executing an uplink signal receivingoperation for a first predetermined period of time according to thetransmission and reception schedule; and, when the uplink signal has notbeen received through the receiving operation but the firstpredetermined period of time has elapsed, continuing the uplink signalreceiving operation for a second predetermined period of time, insteadof transmitting the downlink signal according to the transmission andreception schedule.

A method according to another aspect of the present invention isperformed by a sensor controller that periodically transmits an uplinksignal, and by a stylus that, upon receiving the uplink signal,determines a transmission and reception schedule for a downlink signaland the uplink signal on the basis of the reception timing of the uplinksignal. The method includes: by the stylus, starting an uplink signalreceiving operation according to the transmission and receptionschedule; and, when the uplink signal has not been received through thereceiving operation but the first predetermined period of time haselapsed, transmitting a tone signal, irrespective of content of theunreceived uplink signal.

A stylus according to one aspect of the present invention determines,upon receiving an uplink signal from a sensor controller configured toperiodically transmit the uplink signal, a transmission and receptionschedule for a downlink signal and the uplink signal on the basis of thereception timing of the uplink signal. The stylus executes an uplinksignal receiving operation for a first predetermined period of timeaccording to the transmission and reception schedule, and, when theuplink signal has not been received through the receiving operation butthe first predetermined period of time has elapsed, continues the uplinksignal receiving operation for a second predetermined period of time,instead of transmitting the downlink signal according to thetransmission and reception schedule.

A stylus according to another aspect of the present inventiondetermines, upon receiving an uplink signal from a sensor controllerconfigured to periodically transmit the uplink signal, a transmissionand reception schedule for a downlink signal and the uplink signal onthe basis of the reception timing of the uplink signal. The stylusstarts an uplink signal receiving operation according to thetransmission and reception schedule, and, when the uplink signal has notbeen received through the receiving operation but the firstpredetermined period of time has elapsed, transmits a tone signalirrespective of content of the unreceived uplink signal.

A sensor controller according to one aspect of the present inventionperiodically transmits an uplink signal to a stylus that determines atransmission and reception schedule for a downlink signal and the uplinksignal on the basis of the reception timing of the uplink signal. Thesensor controller includes a counter that indicates an elapsed time fromreception of the last downlink signal, cancels pairing with the styluswhen a value of the counter exceeds a predetermined period of time,determines whether or not content of the downlink signal received fromthe stylus is consistent with a request issued through the uplinksignal, and, when determining that the content of the downlink signalreceived from the stylus is inconsistent with the request issued throughthe uplink signal, resets the counter.

According to one aspect of the present invention, since the possibilitythat the stylus fails to receive the uplink signal can be reduced,occurrence of a situation where the user's writing operation is suddenlydisabled can be avoided.

According to another aspect of the present invention, since transmissionof the tone signal from the stylus is continued even when reception ofthe uplink signal fails, the sensor controller can continuously detectthe position of the stylus, and further, cancellation of the pairing canbe avoided. Accordingly, occurrence of a situation where the user'swriting operation is suddenly disabled can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an electronic device and a stylusaccording to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an inner configuration of a sensorcontroller and a touch sensor illustrated in FIG. 1 ;

FIG. 3A is a mode transition diagram illustrating operation modes of thestylus, and FIG. 3B is a mode transition diagram illustrating operationmodes of the sensor controller;

FIG. 4 is a sequence diagram of operations of the stylus and the sensorcontroller in a case where the stylus newly approaches a touch surface;

FIG. 5 is a sequence diagram of operations of the stylus and the sensorcontroller in a case where the stylus fails to receive an uplink signalwhen both the stylus and the sensor controller are in an operationalmode;

FIG. 6 is a sequence diagram of operations of the stylus and the sensorcontroller in a case where the stylus fails to receive an uplink signalwhen both the stylus and the sensor controller are in an operationalmode;

FIG. 7 is a sequence diagram of operations of the stylus and the sensorcontroller in a case where processing in a communication maintainingmode is executed by the stylus but the stylus S fails to receive anuplink signal;

FIG. 8 is a process flowchart illustrating a process flow in the stylus;

FIG. 9 is a process flowchart illustrating a process flow in the stylus;

FIG. 10 is a process flowchart illustrating a process flow in thestylus;

FIG. 11 is a process flowchart illustrating a process flow in thestylus;

FIG. 12 is a process flowchart illustrating a process flow in the sensorcontroller;

FIG. 13 is a process flowchart illustrating a process flow in the sensorcontroller; and

FIG. 14 is a sequence diagram of operations of the stylus and the sensorcontroller according to a modification of the present embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of the present invention will be explained indetail with reference to the attached drawings.

FIG. 1 is a diagram illustrating an electronic device 2 and a stylus Saccording to an embodiment of the present invention. The electronicdevice 2 is equipped with a tablet computer or a digitizer, for example.As illustrated in FIG. 1 , the electronic device 2 includes a sensorcontroller 20, a touch sensor 21, and a host processor 22. In a casewhere the electronic device 2 is a tablet computer, the electronicdevice 2 is formed with a display.

The sensor controller 20 is an integrated circuit that performsbidirectional communication with the stylus S via the touch sensor 21.The sensor controller 20 is configured to acquire a position, on a touchsurface 2 a, pointed to by the stylus S, to acquire data transmittedfrom the stylus S through the bidirectional communication with thestylus S, and to sequentially supply the position and the data to thehost processor 22, as will be explained in detail later with referenceto FIG. 2 . The sensor controller 20 also executes a process ofacquiring a position, on the touch surface 2 a, pointed to by a finger,and sequentially supplying the position to the host processor 22.

The specific type of the bidirectional communication between the stylusS and the sensor controller 20 is not limited to any particular type.Hereinafter, an explanation will be given on the assumption that activecapacitive communication is utilized. Besides the active capacitivecommunication, electromagnetic resonance (EMR) communication can beutilized, for example. In addition, hereinafter, a signal that istransmitted from the sensor controller 20 to the stylus S is referred toas an uplink signal US, and a signal that is transmitted from the stylusS to the sensor controller 20 is referred to as a downlink signal DS. Anuplink signal US includes a command that indicates what control is to beperformed on the stylus S. A downlink signal DS includes a tone signal.which is a non-modulated carrier wave signal, and a data signal, whichis a modulated carrier wave signal modulated with a predetermined typeof data.

The touch sensor 21 includes a plurality of sensor electrodes disposedin the touch surface 2 a. The details of the touch sensor 21 will beexplained later with reference to FIG. 2 .

The host processor 22 is a central processing unit of the electronicdevice 2, and serves to control various units or portions of theelectronic device 2, and to execute various applications by reading andexecuting a program stored in an internal memory. The variousapplications to be executed by the host processor 22 can include adrawing application. For example, the drawing application is configuredto have a function of generating stroke data on the basis of data (aposition pointed to by the stylus S or a finger, and data transmittedfrom the stylus S) supplied from the sensor controller 20, a function ofgenerating digital ink on the basis of the generated stroke data, and afunction of rendering the generated digital ink to display it on thedisplay.

As illustrated in FIG. 1 , the stylus S includes a processing circuit30, a battery 31, a core body 32, a pen electrode 33, and a pressuresensor 34. The processing circuit 30 is a processor that executesvarious types of processing in the stylus S, which will be explainedlater, by reading and executing a program stored in an internal memory,and is configured to operate with power supplied from the battery 31.The core body 32 is a member forming the pen point of the stylus S. Thepen electrode 33 is a conductive body disposed near the pen tip end ofthe core body 32, and is electrically connected to the processingcircuit 30. The pressure sensor 34 is a sensor that detects a writingpressure value representing a pressure applied to the pen point of thestylus S, and is connected to the rear end of the core body 32. Thewriting pressure value detected by the pressure sensor 34 is supplied tothe processing circuit 30.

The processing circuit 30 receives an uplink signal US via the penelectrode 33, and executes a process of determining, on the basis of thereception timing of the uplink signal US and information included in theuplink signal US, a transmission and reception schedule for a downlinksignal DS and an uplink signal US. The details of this determinationwill be explained later. Further, the processing circuit 30 executes aprocess of generating a downlink signal DS, under the control of acommand included in the uplink signal US, and transmitting the downlinksignal DS via the pen electrode 33. The downlink signal DS thusgenerated is formed of a tone signal T only, or of a tone signal T and adata signal D in accordance with an instruction indicated by the commandincluded in the uplink signal US. Data that is transmitted through thedata signal D can include a pen identification (ID) that is previouslystored in the internal memory of the processing circuit 30, and thelike, in addition to the abovementioned writing pressure value.

FIG. 2 is a diagram illustrating the internal configuration of thesensor controller 20 and the touch sensor 21. As illustrated in FIG. 2 ,the touch sensor 21 includes a plurality of sensor electrodes 21X and aplurality of sensor electrodes 21Y. In a case where the touch surface 2a includes a display surface of the display, the sensor electrodes 21Xor the sensor electrodes 21Y are used also as common electrodes of thedisplay. The electronic device 2 of a type using the sensor electrodes21X or the sensor electrodes 21Y as common electrodes is called an“in-cell type,” for example. On the other hand, the electronic device 2of a type including a common electrode of the display separately fromthe sensor electrodes 21X and 21Y is called an “out-cell type” or an“on-cell type,” for example. The following explanation is based on theassumption that the electronic device 2 is an in-cell type. However, thepresent invention is also applicable to an electronic device of anout-cell type or an on-cell type.

In a case where the display executes a pixel driving process, it isnecessary to maintain the potential of each common electrode at apredetermined common potential Vcom. Thus, in the electronic device 2that is an in-cell type, the sensor controller 20 cannot communicatewith the stylus S or detect any finger when the display is executing thepixel driving process. Therefore, the sensor controller 20 is configuredto communicate with the stylus S and detect a finger by using ahorizontal blanking interval and a vertical blanking interval duringwhich the pixels are not driven. Specifically, regarding a displayperiod of one screen as one frame, in which a horizontal blankinginterval constitutes one time slot and a vertical blanking intervalconstitutes one time slot, the sensor controller 20 is configured tocommunicate with the stylus S and detect a finger within these timeslots.

As illustrated in FIG. 2 , the sensor controller 20 includes a microcontroller unit (MCU) 50, a logic unit 51, transmission units 52 and 53,a reception unit 54, a selection unit 55, and a counter 58.

The MCU 50 and the logic unit 51 are control units that control atransmission and reception operation of the sensor controller 20 bycontrolling the transmission units 52 and 53, the reception unit 54, andthe selection unit 55. Specifically, the MCU 50 is a microprocessor thathas an internal read-only memory (ROM) and an internal random accessmemory (RAM), and operates by executing programs stored in the ROM andthe RAM. The MCU 50 also has a function of outputting a common potentialVcom and a command COM. The command COM corresponds to a commandincluded in an uplink signal US. On the other hand, the logic unit 51 isconfigured to output control signals ctrl_t, ctrl_r, sTR, selX, andselY, under the control of the MCU 50.

A command COM which is outputted from the MCU 50 may include, forexample, information that specifies a set of information relating to atransmission and reception schedule, information indicating a time slotand a frequency allocated to the stylus S, information indicating thedetails of data to be transmitted through the data signal D from thestylus S, and the like. Here, the set of information relating to atransmission and reception schedule may be one of information setsregarding a transmission and reception schedule previously shared by thesensor controller 20 and the stylus S. Each of these information setsmay include, for example, information regarding the time length (=cycleUpIntv) of one frame, and information regarding arrangement of timeslots in one frame.

The MCU 50 may previously store a set of information relating to atransmission and reception schedule according to the display of theelectronic device 2, and may include, in the command COM, informationthat specifies the set of information. In addition, the MCU 50 maydetermine, on the basis of the number of paired styluses S, one or moretime slots and a frequency to be allocated to each stylus S, and includethe time slots and the frequency in the command COM. As a result, thesensor controller 20 can simultaneously be paired with a plurality ofstyluses S by time division multiplexing or frequency divisionmultiplexing, for example. However, the following explanation is basedon the assumption that the sensor controller 20 communicates with onestylus S only. The transmission unit 52 is a circuit that generates afinger detection signal FDS for detecting a finger, under the control ofthe MCU 50. The finger detection signal FDS is formed of K pulse trainseach including K pulses (data representing “1” or “−1”), for example. Kis equal to the number of the sensor electrodes 21Y. Further, the Kpulse trains include different contents (i.e., combination of the Kpulses).

The transmission unit 53 is a circuit that generates an uplink signal USon the basis of a command COM supplied from the MCU 50 and a controlsignal ctrl_t transmitted from the logic unit 51. Specifically, thetransmission unit 53 generates an uplink signal US by adding apredetermined preamble to the head of a command COM supplied from theMCU 50, spreading the resultant symbol train with a predetermined spreadcode (e.g., an 11-chip length spread code having an autocorrelationcharacteristic), and further modulating the train through a cyclicshift, for example.

The selection unit 55 includes a switch 56 and conductor selectingcircuits 57 x and 57 y.

The switch 56 is a switch element that is configured to connect a commonterminal to any one of a T1 terminal, a T2 terminal, a D terminal, andan R terminal. The T2 terminal is actually a set of terminals as many asthe sensor electrodes 21Y. The common terminal in the switch 56 isconnected to the conductor selecting circuit 57 y. The T1 terminal isconnected to an output end of the transmission unit 53. The T2 terminalis connected to an output end of the transmission unit 52. The Dterminal is connected to an output end of the MCU 50 from which thecommon potential Vcom is outputted. The R terminal is connected to aninput end of the reception unit 54.

The conductor selecting circuit 57 x is a switch element for selectivelyconnecting the plurality of sensor electrodes 21X to the input end ofthe reception unit 54. The conductor selecting circuit 57 x isconfigured to be able to connect at least one or all of the sensorelectrodes 21X simultaneously to the input end of the reception unit 54.

The conductor selecting circuit 57 y is a switch element for selectivelyconnecting the plurality of sensor electrodes 21Y to the common terminalof the switch 56. The conductor selecting circuit 57 y is configured tobe able to connect at least one or all of the sensor electrodes 21Ysimultaneously to the common terminal of the switch 56. In addition,when the T2 terminal and the common terminal are connected in the switch56, the conductor selecting circuit 57 y connects the terminalsconstituting the T2 terminal and the sensor electrodes 21Y in one-to-onerelation.

Three control signals sTR, selX, and selY are supplied from the logicunit 51 to the selection unit 55. Specifically, the control signal sTR,the control signal selX, and the control signal selY are supplied to theswitch 56, the conductor selecting circuit 57 x, and the conductorselecting circuit 57 y, respectively. By controlling the selection unit55 with use of the control signals sTR, selX, and selY, the logic unit51 performs transmission of an uplink signal US or a finger detectionsignal FDS, application of a common potential Vcom, and reception of adownlink signal DS or a finger detection signal FDS.

To transmit uplink signals US, the logic unit 51 controls the selectionunit 55 to simultaneously connect all the sensor electrodes 21Y to theinput end of the transmission unit 53. As a result, uplink signals USare transmitted from the entirety of the touch surface 2 a.

To receive a downlink signal DS, the logic unit 51 performs control indifferent ways, in a case (global scanning) of receiving a downlinksignal DS in order to detect an undetected stylus S, and in a case(local scanning) of receiving a downlink signal DS from a detectedstylus S. A specific explanation thereof will be given herein. First, inthe case of conducting global scanning, the logic unit 51 controls theselection unit 55 to connect sequentially all the sensor electrodes 21Xand 21Y to the input end of the reception unit 54. Next, in the case ofconducting local scanning, at a time of receiving a tone signal T, thelogic unit 51 controls the selection unit 55 to sequentially connect afew sensor electrodes 21X and 21Y that are located around a positionpointed to by the stylus S to the input end of the reception unit 54.Next, at a time of receiving a data signal D, the logic unit 51 controlsthe selection unit 55 to connect the sensor electrode 21X or sensorelectrode 21Y that is closest to the position pointed to by the stylusS, to the input end of the reception unit 54.

In the case of transmitting and receiving a finger detection signal FDS,the logic unit 51 controls the selection unit 55 to connect theterminals constituting the T2 terminal of the switch 56 to the sensorelectrodes 21Y in one-to-one relation. Then, while maintaining thisstate, the logic unit 51 controls the selection unit 55 to sequentiallyselect the plurality of sensor electrodes 21X one by one, and connectthe selected sensor electrodes 21X to the reception unit 54.

As can be understood from the abovementioned controls by the logic unit51, communication with the stylus S and detection of a position pointedto by a finger are performed by time division. The following explanationfocuses on communication with the stylus S only. However, actualcommunication with the stylus S is carried out in between interveningoperations of detecting a position pointed to by a finger.

In a case of applying a common potential Vcom, the logic unit 51controls the selection unit 55 to simultaneously connect all the sensorelectrodes 21Y to the D terminal of the switch 56. As a result, thepotential of each of the sensor electrodes 21Y becomes equal to thecommon potential Vcom.

The reception unit 54 is a circuit that receives, on the basis of thecontrol signal ctrl_r from the logic unit 51, a finger detection signalFDS transmitted from the transmission unit 52 and a downlink signal DStransmitted from the stylus S. At a timing of receiving the fingerdetection signal FDS, the reception unit 54 acquires K current valuesfor the respective sensor electrodes 21X, and calculates, for each ofthe abovementioned K pulse trains, an inner product of K pulsesconstituting the pulse train and the acquired K current values, therebycalculating a detection level at each of intersections between thesensor electrodes 21X and the sensor electrodes 21Y. Then, on the basisof the result of the calculation, the reception unit 54 determines atouched region (touch region) of the touch surface 2 a and outputs theregion to the host processor 22 via the MCU 50.

At a timing of receiving the downlink signal DS, the reception unit 54derives a position pointed to by the stylus S on the basis of thereception intensities of tone signals T at the sensor electrodes 21X and21Y, and acquires data transmitted from the stylus S by demodulating adetected data signal D. Then, the reception unit 54 outputs the derivedpointed position and the acquired data to the host processor 22 via theMCU 50.

The counter 58 is connected to the MCU 50 and stores an elapsed timefrom a reset time point. The MCU 50 is configured to, upon receiving adownlink signal DS, reset the counter 58. Accordingly, the elapsed timerecorded in the counter 58 represents a time elapsed from the receptiontime of the last downlink signal DS at the MCU 50.

FIG. 3A is a mode transition diagram illustrating the operation modes ofa stylus S. FIG. 3B is a mode transition diagram illustrating theoperation modes of the sensor controller 20.

First, attention is given to FIG. 3A. The stylus S is configured tooperate in any one of a discovery mode, an operational mode, and acommunication maintaining mode. The discovery mode is a mode in whichonly reception of an uplink signal US is continuously or intermittentlyconducted. After receiving an uplink signal US in the discovery mode,the stylus S is shifted to the operational mode.

The operational mode is a mode in which transmission of a downlinksignal DS and reception of a next uplink signal US are conducted on thebasis of a transmission and reception schedule determined on the basisof an uplink signal US received so far. After failing to receive anuplink signal US in the operational mode (i.e., conducting an uplinksignal US receiving operation but failing to receive any uplink signalUS), the stylus S is shifted to the communication maintaining mode.

The communication maintaining mode is a mode in which communication withthe sensor controller 20 is maintained for a while even when receptionof an uplink signal US fails. The details of specific operation of thestylus S in the communication maintaining mode will be described later.When no uplink signal US has been received but the communicationmaintaining mode times out, the stylus S is shifted to the discoverymode. On the other hand, when an uplink signal US is received in thecommunication maintaining mode, the stylus S returns to the operationalmode.

Next, attention is given to FIG. 3B. The sensor controller 20 isconfigured to operate in any one of a discovery mode, a pairingexecuting mode, and an operational mode. In any mode, the sensorcontroller 20 performs an uplink signal US transmitting operation in afixed cycle and performs a downlink signal DS receiving operation duringan interval between the cycles. The uplink signal US transmission cyclemay be formed of the one frame described above, for example.

The discovery mode is a mode in which pairing with the stylus S has notbeen established, and in which the sensor controller 20 conducts theabovementioned global scanning. An uplink signal US that is transmittedin the discovery mode includes information that specifies a set ofinformation relating to a transmission and reception schedule, andallocation of time slots and a frequency. After receiving the uplinksignal US, the stylus S determines a transmission and reception schedulefor a downlink signal DS and an uplink signal US, and transmits a tonesignal T in each allocated time slot. After receiving a downlink signalDS in the discovery mode, the sensor controller 20 is shifted to thepairing executing mode.

The pairing executing mode is a mode in which pairing is establishedwith the stylus S from which the downlink signal DS has beentransmitted. The sensor controller 20, upon entering the pairingexecuting mode, first conducts global scanning to detect a positionpointed to by the stylus S, and then, conducts local scanning. Duringthis local scanning, the sensor controller 20 transmits an uplink signalUS that includes an instruction to transmit a pen ID. Upon receiving theuplink signal US, the stylus S sequentially transmits a tone signal T,the pen ID, and a data signal D, by using the time slots and thefrequency allocated by the previous uplink signal US. By demodulatingthe received data signal D, the sensor controller 20 acquires the pen IDand stores the pen ID in the internal memory so as to establish pairingwith the stylus S. Then, the sensor controller 20 is shifted to theoperational mode. In addition, the sensor controller 20 derives theposition pointed to by the stylus S on the basis of the reception resultof the tone signal T and outputs the derived position and the acquiredpen ID to the host processor 22.

The operational mode is a mode in which local scanning is repeatedlyconducted. The sensor controller 20, upon entering the operational mode,derives a position pointed to by the stylus S on the basis of thereception result of a tone signal T that is received after transmissionof an uplink signal US, acquires data transmitted from the stylus S bydemodulating the next received data signal D, and sequentially outputsthe derived point position and the acquired data to the host processor22. Moreover, when the elapsed time recorded in the counter 58 exceeds apredetermined period of time, the sensor controller 20 cancels thepairing with the stylus S, and is shifted to the discovery mode.

Next, operations of the stylus S and the sensor controller 20 accordingto the present embodiment will be explained in more detail withreference to sequence diagrams in FIGS. 4 to 7 .

First, FIG. 4 is a sequence diagram of operations of the stylus S andthe sensor controller 20 in a case where the stylus S newly approachesthe touch surface 2 a. In this example, the stylus S is powered on attime t1, a pen down (a state in which the stylus S becomes communicablewith the sensor controller 20) is conducted at time t2, and a pen touch(a state in which the pen point of the stylus S is brought into contactwith the touch surface 2 a) is conducted at time t3.

The sensor controller 20 is configured to first transmit an uplinksignal US in a predetermined cycle UpIntv, and to receive a downlinksignal DS during an uplink signal US transmission interval. It is to benoted that reception periods each having a time length P5 and beingdenoted by “R” in FIG. 4 correspond to the abovementioned time slots.Since the sensor controller 20 is in the discovery mode in these timeslots, the first uplink signal US in FIG. 4 includes information thatspecifies a set of information relating to a transmission and receptionschedule and allocation of time slots and a frequency.

After being powered on, the stylus S continuously or intermittentlyexecutes the uplink signal US receiving operation. Then, after enteringa pen down state at time t2, the stylus S becomes able to receive anuplink signal US. In FIG. 4 , the stylus S receives an uplink signal USat time t4. It is to be noted that, in FIG. 4 and the followingdrawings, thick circles each indicate that the stylus S successfullyreceives an uplink signal US. Further, after the stylus S enters a pentouch state at time t3, a writing pressure value which is detected bythe pressure sensor 34 becomes a value greater than 0.

After receiving the uplink signal US at time t4, the stylus S determinesa transmission and reception schedule for a downlink signal DS and thenext uplink signal US on the basis of the reception timing of the uplinksignal US and information included in the uplink signal US. Thedetermined transmission and reception schedule indicates that a tonesignal T is transmitted in each time slot, and further, the next uplinksignal US receiving operation is executed using the last predeterminedperiod of time P1 of one cycle, as illustrated in FIG. 4 . Thepredetermined period of time P1 may be equal to the predetermined periodof time P5, or may be longer than the predetermined period of time P5,as illustrated in FIG. 4 . After determining the transmission andreception schedule, the stylus S makes an entry into the operationalmode.

After receiving, at time t5, a tone signal T first transmitted from thestylus S according to the transmission and reception schedule, thesensor controller 20 makes an entry into the pairing executing mode andconducts global scanning additionally using subsequently received tonesignals T. As a result, the sensor controller 20 derives the initialposition of the stylus S. Thereafter, the sensor controller 20 transmitsan uplink signal US that includes a command including an instruction totransmit the pen ID.

After receiving the uplink signal US at time t6, the stylus S updatesthe transmission and reception schedule for a downlink signal DS and thenext uplink signal US on the basis of the reception timing of the uplinksignal US and information included in the uplink signal US. The updatedtransmission and reception schedule indicates that a tone signal T isfirst transmitted, and then, a data signal D is transmitted, and thenext uplink signal US receiving operation is executed by using the lastpredetermined period of time P1 in one cycle, as illustrated in FIG. 4 .

The sensor controller 20 derives the position of the stylus S byconducting local scanning using the tone signal T transmitted from thestylus S. Next, the sensor controller 20 acquires the pen ID transmittedfrom the stylus S by demodulating a data signal D transmitted from thestylus S, and stores the pen ID in the internal memory. As a result, thesensor controller 20 establishes pairing and is shifted to theoperational mode.

FIG. 5 is a sequence diagram of operations of the stylus S and thesensor controller 20 in a case where the stylus S fails to receive anuplink signal US when both the stylus S and the sensor controller 20 arein the operational mode. In FIG. 5 and the following drawings, thick xmarks each indicate that the stylus S fails to receive an uplink signalUS. In this example, the stylus S starts an uplink signal US receivingoperation at time t10 according to the transmission and receptionschedule, but has received no uplink signal US before time t11 at whichthe predetermined period of time P1 has elapsed.

In this case, the stylus S makes an entry into the communicationmaintaining mode and continues the uplink signal US receiving operationfor a predetermined period of time P2 that follows the predeterminedperiod of time P1. The time length of the predetermined period of timeP2 is not limited to a particular time length, but may be preferably setto be ¼ of the cycle UpIntv, for example. Accordingly, even in a casewhere the uplink signal US receiving operation is shifted forward forsome reason, reception of an uplink signal US is possible. Therefore,the possibility that the stylus S may fail to receive an uplink signalUS can be reduced.

Next, the stylus S continuously transmits a tone signal T for apredetermined period of time P3 before a timing of starting the nextuplink signal US receiving operation (i.e., a timing between the end ofthe cycle UpIntv and the start of the predetermined period of time P1)according to the transmission and reception schedule, irrespective ofthe content of an uplink signal US that the stylus S has failed toreceive. The time length of the predetermined period of time P3 is alsonot limited to a particular length, but may be preferably set to be ½ ofthe cycle UpIntv, for example.

Here, the sensor controller 20 is configured to reset the counter 58illustrated in FIG. 2 not only in a case where the content of thedownlink signal DS received from the stylus S is consistent with therequest issued through the uplink signal US, but also in a case wherethe content of the downlink signal DS received from the stylus S isinconsistent with the request issued through the uplink signal US.Accordingly, even when the stylus S is in the communication maintainingmode, the operational mode is maintained as the mode of the sensorcontroller 20 as long as a tone signal T from the stylus S arrives atthe sensor controller 20. Further, the sensor controller 20 isconfigured to derive the position of the stylus S by using a downlinksignal DS received from the stylus S, even in a case where the contentof the downlink signal DS is inconsistent with the request issuedthrough the uplink signal US. Accordingly, the sensor controller 20continuously derives the position of the stylus S as long as a tonesignal T from the stylus S arrives.

After completing transmission of the tone signal T, the stylus Sexecutes an uplink signal US receiving operation for a predeterminedperiod of time P4. The time length of the predetermined period of timeP4 may be set to a value obtained by subtracting a total value of thepredetermined period of time P2 and the predetermined period of time P3from the cycle UpIntv. In one example, when the predetermined period oftime P2 and the predetermined period of time P3 are set to be ¼ of thecycle UpIntv and ½ of the cycle UpIntv, respectively, as explainedabove, the predetermined period of time P4 may be set to ¼ of the cycleUpIntv. It is to be noted that the predetermined period of time P4 maybe set to be longer than the predetermined period of time P1. In thiscase, the stylus S starts the uplink signal US receiving operation priorto the timing of starting the uplink signal US receiving operationdetermined according to the transmission and reception schedule.

When performing the receiving operation for the predetermined period oftime P4 but having received no uplink signal US, the stylus S repeatsthe abovementioned operations from the uplink signal US receivingoperation for the predetermined period of time P2. In this case, theuplink signal US receiving operation for the predetermined period oftime P2 is executed so as to follow the uplink signal US receivingoperation for the immediately preceding predetermined period of time P4.

In the example in FIG. 5 , the stylus S successfully receives an uplinksignal US at time t12. In response to this, the stylus S updates thetransmission and reception schedule and returns to the operational mode.

FIG. 6 is a sequence diagram of operations of the stylus S and thesensor controller 20 in a case where the stylus S fails to receive anuplink signal US when both the stylus S and the sensor controller 20 arein the operational mode, as in FIG. 5 . The example in FIG. 6 differsfrom that in FIG. 5 in that, in FIG. 6 , a pen up occurs at time t13when the stylus S is in the communication maintaining mode.

The stylus S is configured to, after entering the communicationmaintaining mode, perform an uplink signal US receiving operation,instead of a tone signal T transmitting operation, in a case where thewriting pressure value detected by the pressure sensor 34 indicates thatthe pen point is not in contact with the touch surface 2 a. As a result,transmission of a tone signal T is not performed by the stylus S, butthe uplink signal US receiving operation is continuously performed aftertime t13 in FIG. 6 . Accordingly, the sensor controller 20 discontinuesthe operation of deriving the position of the stylus S and returns tothe discovery mode at an early stage, but the pen-up state of the stylusS means that writing is not being executed so it is unlikely that theuser will experience any inconvenience. Also, the period of time of theuplink signal US receiving operation becomes long, compared to a casewhere transmission of a tone signal T is performed, and thus, it is morelikely that an uplink signal US is successfully received.

FIG. 7 is a sequence diagram of operations of the stylus S and thesensor controller 20 in a case where the stylus S executes processing ofthe communication maintaining mode but fails to receive an uplink signalUS. The stylus S is shifted to the discovery mode at time t21 when thenumber of times a series of processing in the communication maintainingmode is executed (the uplink signal US receiving operation for thepredetermined period of time P2, the tone signal T transmittingoperation for the predetermined period of time P3, and the uplink signalUS receiving operation for the predetermined period of time P4) reachesa predetermined time while no uplink signal US has been received. Then,the processing of the communication maintaining mode is terminated. Inaddition, the sensor controller 20 resets the counter 58 illustrated inFIG. 2 at time t20 when the last tone signal T is received. Thereafter,at time t22 when the elapsed time recorded in the counter 58 exceeds apredetermined period of time P6, the sensor controller 20 cancelspairing with the stylus S, and returns to the discovery mode.

Next, operations of the stylus S and the sensor controller 20 accordingto the present embodiment will be explained in more detail withreference to process flowcharts illustrated in FIGS. 8 to 13 .

FIGS. 8 to 11 are flow diagrams each illustrating a process flow in thestylus S. FIGS. 8 to 11 each illustrate a process which is executed bythe processing circuit 30 illustrated in FIG. 1 .

With reference to FIG. 8 first, the powered-on stylus S makes an entryinto the discovery mode (step S1), and starts an uplink signal USreceiving operation (step S2). Then, whether or not an uplink signal USis received in the receiving operation is determined (step S3). Thestylus S repeats step S3 until reception of an uplink signal US isdetermined. When the reception is determined, the uplink signal USreceiving operation is stopped (step S4). It is to be noted that thestylus S may continuously execute the uplink signal US receivingoperation which is started in step S2, or may intermittently execute thereceiving operation. In this case, in order to ensure reception of anuplink signal US when the stylus S enters a signal-receivable area, itis preferable to set the duration of one receiving operation to belonger than the abovementioned cycle UpIntv.

Next, on the basis of the repletion timing of the uplink signal US andinformation included in the uplink signal US, the stylus S havingreceived the uplink signal US determines a transmission and receptionschedule for a downlink signal DS and the next uplink signal US (stepS5). Then, the stylus S enters the operational mode (step S6).

According to the determined transmission and reception schedule, thestylus S in the operational mode intermittently determines whether ornot a timing of transmitting a downlink signal DS has arrived (stepS10), and whether or not a timing of receiving an uplink signal US hasarrived (step S11), as illustrated in FIG. 9 .

After determining, in step S10, that the timing of transmitting adownlink signal DS has arrived, the stylus S starts transmission of adownlink signal DS including content consistent with the indication inthe uplink signal US (step S12). A downlink signal DS that istransmitted in this manner is the abovementioned tone signal T or datasignal D. The time length of this signal is previously adjusted suchthat the transmission is completed within a period of time (i.e., theabovementioned time slot) indicated by the transmission and receptionschedule. Subsequently, the stylus S determines whether or nottransmission of the downlink signal DS is completed (step S13). When thecompletion is determined, the process returns to the determinations insteps S10 and S11.

When determining, in step S11, that the timing of receiving an uplinksignal US has arrived, the stylus S starts an uplink signal US receivingoperation (step S14). Then, the stylus S determines whether or not anuplink signal US is received (step S15). When determining that thereception is not done, the stylus S determines whether or not thepredetermined period of time P1 has elapsed from the start of thereceiving operation (step S16). When it is determined that thepredetermined period of time P1 has not elapsed, the process returns tostep S15 to continue the determination process. When it is determinedthat the predetermined period of time P1 has elapsed, the processproceeds to step 30 in FIG. 10 .

After determining, in step S15, that the uplink signal US is received,the stylus S stops the uplink signal US receiving operation (step S18),and updates the transmission and reception schedule for a downlinksignal DS and the next uplink signal US on the basis of the receptiontiming of the uplink signal US received here and information included inthe uplink signal US (step S19). Then, the stylus S acquirestransmission data in accordance with a command included in the uplinksignal US (step S20), and the process returns to the determination insteps S10 and S11. The data acquired in step S20 is collectively ordividedly included in a data signal D to be transmitted in the followingstep S12.

In step S30 in FIG. 10 , the stylus S enters the communicationmaintaining mode. Then, the stylus S determines the abovementionedpredetermined periods of time P2, P3, and P4 on the basis of the latesttransmission and reception schedule determined in step S5 or step S19(step S31). In a typical case, P2=UpIntv/4, P3=UpIntv/2, and P4=UpIntv/4may be set on the basis of the cycle UpIntv indicated by thetransmission and reception schedule.

Next, the stylus S substitutes 1 for a variable n that represents thenumber of times the series of processing in the communicationmaintaining mode is executed (the uplink signal US receiving operationfor the predetermined period of time P2, the tone signal T transmittingoperation for the predetermined period of time P3, and the uplink signalUS receiving operation for the predetermined period of time P4) (stepS32), and determines whether or not the writing pressure value detectedby the pressure sensor 34 is 0 (step S33).

When determining, in step S33, that the writing pressure value is 0, thestylus S repeatedly determines whether or not an uplink signal US isreceived, until the predetermined periods of time P2+P3+P4 elapse (stepsS34 and S35). After the stylus S determines, in step S34, that an uplinksignal US is received, the process proceeds to step S17 in FIG. 9 . Asillustrated in FIG. 9 , the stylus S enters the operational mode in stepS17, and the process proceeds to step S18. Thereafter, the stylus Soperates in the operational mode. On the other hand, when determining,in step S35, that the predetermined periods of time P2+P3+P4 haveelapsed, the stylus S increments the variable n by 1 (step S36), andcompares the variable n with a predetermined value N (step S37). In acase where the result illustrates n>N, the process returns to step S1 inFIG. 8 . Accordingly, the stylus S returns to the discovery mode. On theother hand, in a case where n>N is not established, the process returnsto step S33 at which the stylus S continues the processing of thecommunication maintaining mode.

When determining, in step S33, that the writing pressure value is not 0,the stylus S repeatedly determines whether or not an uplink signal US isreceived until the predetermined period of time P2 elapses (steps S40and S41), as illustrated in FIG. 11 . After the stylus S determines, instep S40, that an uplink signal US is received, the process returns tostep S17 in FIG. 9 . The following steps have been previously explained.On the other hand, when determining, in step S41, that the predeterminedperiod of time P2 has elapsed, the stylus S stops the uplink signal USreceiving operation (step S42), and starts transmission of a tone signalT (step S43). Subsequently, the stylus S continues the transmission of atone signal T. When the predetermined period of time P3 has elapsed fromthe stop of the uplink signal US receiving operation in step S42 (stepS44), the transmission of a tone signal T is stopped (step S45).

After stopping the transmission of a tone signal T, the stylus S startsan uplink signal US receiving operation again (step S46), and then,repeatedly determines whether or not an uplink signal US is receiveduntil the predetermined period of time P4 elapses (steps S47 and S48).After the stylus S determines, in step S47, that an uplink signal US isreceived, the process proceeds to step S17 in FIG. 9 . The followingsteps have been previously explained. On the other hand, afterdetermining, in step S48, that the predetermined period of time P4 haselapsed, the stylus S increments the variable n by 1 (step S49), andcompares the variable n with the predetermined value N (step S50). Whenthe result illustrates that n>N, the process returns to step S1 in FIG.8 . Accordingly, the stylus S returns to the discovery mode. On theother hand, when n>N is not established, the process returns to step S33in FIG. 10 in which the stylus S continues the processing of thecommunication maintaining mode.

Next, FIGS. 12 and 13 are process flow diagrams each illustrating aprocess flow in the sensor controller 20. FIGS. 12 and 13 eachillustrate a process which is executed by the MCU 50 illustrated in FIG.2 .

Referring to FIG. 12 first, the powered-on sensor controller 20 entersthe discovery mode (step S60), and determines a transmission andreception schedule for an uplink signal US and a downlink signal DS(step S61). This determination may be made by allocating transmission ofan uplink signal US and reception of a downlink signal DS to respectivetime slots in one frame, for example.

Next, the sensor controller 20 continuously determines whether or not atiming of transmitting an uplink signal US has arrived (step S62), andwhether or not a timing of receiving a downlink signal DS has arrived(step S63), according to the determined transmission and receptionschedule.

When determining, in step S62, that the timing of transmitting an uplinksignal US has arrived, the sensor controller 20 starts transmission ofan uplink signal US (step S64). After the transmission of an uplinksignal US is completed, the process returns to the determinations insteps S62 and S63.

When determining, in step S63, that a timing of receiving a downlinksignal DS has arrived, the sensor controller 20 starts a downlink signalDS receiving operation (step S66). Then, the sensor controller 20repeatedly determines whether or not a downlink signal DS is receiveduntil the predetermined period of time P5 that is as long as the timelength of one time slot elapses (steps S67 and S68).

After the sensor controller 20 determines, in step S67, that a downlinksignal DS is received, the process proceeds to step S80 in FIG. 13 .After determining, in step S68, that the predetermined period of time P5has elapsed, the sensor controller 20 determines whether or not theelapsed time indicated by the counter 58 illustrated in FIG. 2 hasreached the predetermined period of time P6 (step S69). When the elapsedtime has not reached the predetermined period of time P6, the processreturns to the determinations in steps S62 and S63. When the elapsedtime has reached the predetermined period of time P6, the currentoperation mode is determined (step S70). When the current operation modeis the discovery mode, the process returns to the determinations insteps S62 and S63. When the current operation mode is any one of theremaining modes, pairing is canceled if the pairing is established, andthe sensor controller 20 enters the discovery mode (step S71). Then, theprocess returns to the determinations in steps S62 and S63.

Referring to FIG. 13 , the sensor controller 20 determines, in step S80,whether or not the received downlink signal DS is consistent with therequest issued through the uplink signal US (step S80). In one example,in a case where a command included in the uplink signal US indicates aninstruction to transmit a tone signal T in the first two time slots andto transmit a data signal D including a writing pressure value in thefollowing time slot, when the content of the received downlink signal DSis consistent with the instruction, the determination result in step S80is positive, and, when the content is not consistent, the determinationresult in step S80 is negative.

After obtaining the positive result in step S80, the sensor controller20 resets the counter 58 (step S81), and determines the currentoperation mode (step S82). When the result illustrates that the currentoperation mode is the discovery mode, the sensor controller 20 entersthe pairing mode (step S83). Then, the process returns to thedeterminations in steps S62 and S63.

When the current operation mode is the pairing mode, pairing processingis executed (step S84). This pairing processing includes determining acommand to be included in an uplink signal US that is transmitted next,and also includes various types of processing (e.g., position derivingprocessing based on the result of global scanning and local scanning,processing to output the derived position to the host processor 22,processing to acquire the pen ID transmitted from the stylus S, etc.)based on a downlink signal DS transmitted from the stylus S in responseto the command. Subsequently, the sensor controller 20 determineswhether or not pairing is established as the result of the pairingprocessing (step S85). The determination result is positive whenreception of a pen ID is completed, and the determination result isnegative when reception of a pen ID is not completed.

After obtaining the positive result in step S85, the sensor controller20 enters the operational mode (step S86), and then, the process returnsto the determinations in steps S62 and S63. After obtaining the negativeresult in step S85, the sensor controller 20 remains in the pairingmode, and the process returns to the determinations in steps S62 andS63.

When determining, in step S82, that the current operation mode is theoperational mode, the sensor controller 20 derives the position of thestylus S and acquires transmission data from the stylus S on the basisof the received downlink signal DS (step S87). It is to be noted that,in a case where reception of tone signals T in a plurality of time slotsis required to derive the position of the stylus S, the sensorcontroller 20 may store the reception intensities of tone signals T atthe respective sensor electrodes 21X and 21Y in respective time slots,and derive the position on the basis of the stored reception intensitiesafter reception in the last time slot is finished. Also, in a case wherereception of data signals D in a plurality of time slots is required toacquire transmission data from the stylus S, the sensor controller 20may store a piece of the transmission data in each time slot, andcollectively acquire the stored pieces as one transmission data setafter reception in the last time slot is finished. The sensor controller20 outputs the derived position and the acquired transmission data tothe host processor 22 (step S88). Then, the process returns to thedeterminations in steps S62 and S63.

After obtaining the negative result in step S80, the sensor controller20 resets the counter 58 (step S90), as in the case where the positiveresult is obtained. Accordingly, even in a case where a signalconsistent with the request is not transmitted from the stylus S, thepairing with the stylus S is maintained. It is to be noted that thecounter 58 is reset at each of steps S81 and S90 in FIG. 13 , but thecounter 58 may be reset prior to the determination in step S80.

Next, the sensor controller 20 derives the position of the stylus S onthe basis of the received signal (tone signal T) (step S91) and outputsthe derived position to the host processor 22 (step S92). In a casewhere, also in step S91, reception of tone signals T in a plurality oftime slots is required to derive the position of the stylus S, thesensor controller 20 may store the reception intensities of tone signalsT at the respective sensor electrodes 21X and 21Y in the respective timeslots, and derive the position on the basis of the stored receptionintensities after reception in the last time slot is finished. Further,in step S92, the sensor controller 20 may output not only the derivedposition but also the writing pressure value (the latest writingpressure value acquired during the operational mode of the stylus S)obtained so far, to the host processor 22. Thereafter, the process inthe sensor controller 20 returns to the determinations in steps S62 andS63.

As explained so far, with the stylus S and the sensor controller 20according to the present embodiment, the possibility that the stylus Sfail to receive an uplink signal US can be reduced. Accordingly,occurrence of a situation where the user's writing operation is suddenlydisabled can be avoided.

In addition, with the stylus S and the sensor controller 20 according tothe present embodiment, even if reception of an uplink signal US fails,transmission of a tone signal T from the stylus S is continued.Therefore, the sensor controller 20 can continuously detect the positionof the stylus S, and further, cancellation of pairing can be avoided.Consequently, occurrence of a situation where the user's writingoperation is suddenly disabled can be avoided.

In addition, with the stylus S and the sensor controller 20 according tothe present embodiment, in a case where the writing pressure value >0,not a data signal D including the writing pressure value but a tonesignal T is transmitted. Thus, compared to a modulation wave (e.g.,modulation wave obtained through on-off keying (OOK), amplitude-shiftkeying (ASK), or phase-shift keying (PSK) modulation) formed bymodulating a tone signal on the basis of the writing pressure value, acontinuous signal is obtained or the frequency is fixed so that thedetection possibility of the sensor controller 20 can be enhanced.Consequently, in a noisy environment where no uplink signal US can bedetected, the possibility that the sensor controller 20 successfullydetects the stylus S can be increased. It is to be noted that, when thestylus S is transmitting a tone signal T instead of a data signal Dincluding the writing pressure value, the sensor controller 20 mayoutput the last acquired writing pressure value as the current writingpressure value as well as the position obtained during this period oftime, to the host processor 22.

The preferred embodiment of the present invention has been describedabove. However, the present invention is not limited to the aboveembodiment, and various embodiments of the present invention can beimplemented within the scope of the disclosure.

For example, in a case where the writing pressure value >0, not a datasignal D including the writing pressure value but a tone signal T istransmitted from the stylus S in the embodiment described above, but, ina case where the writing pressure value >0, a data signal D (signalformed by modulation based on the writing pressure value) including thewriting pressure value may be transmitted. In this case, the possibilitythat the sensor controller 20 detects the stylus S is lowered, comparedto the case where the tone signal T is transmitted. However, since thesensor controller 20 in normal operation is likely requesting normalcommunication of the stylus S (e.g., transmission of a data signal Dincluding the writing pressure value), it is possible to continue normaloperation of the system as a whole.

Irrespective of whether a data signal D is transmitted or a tone signalT is transmitted in the communication maintaining mode, as viewed fromthe host processor 22, the stylus S and the sensor controller 20 appearto operate normally even in a case where the stylus S actually fails todetect an uplink signal US. Thus, a robust system including the stylus Sand the sensor controller 20 can be provided. Therefore, it becomespossible to prevent occurrence of line cut, which is annoying to a user,which results from the stylus S's failure to detect an uplink signal US,which is not necessary for the purpose of detecting the position of thestylus S.

FIG. 14 is a sequence diagram of operations of the stylus S and thesensor controller 20 according to a modification of the presentembodiment. The stylus S according to the present modification differsfrom the stylus S according to the embodiment described above in that,in step S31 in FIG. 10 , the stylus S according to the presentmodification determines the predetermined periods of time P2, P3, and P4on the basis of a cycle UpIntv′ the value of which is different from thecycle UpIntv. It is to be noted that a case where UpIntv′<UpIntv isillustrated in FIG. 14 , but UpIntv′>UpIntv may be set. Moreover, FIG.14 illustrates an example where P2=UpIntv′/4, P3=UpIntv′/2, andP4=UpIntv′/4, but any one or more of the predetermined periods of timeP2, P3, and P4 may be set to a value different from that of theembodiment described above. Accordingly, in a case where the stylus Scannot receive an uplink signal US because a timing for the stylus S toexecute the uplink signal US receiving operation deviates from a timingfor the sensor controller 20 to execute the uplink signal UStransmitting operation, the stylus S can receive an uplink signal USafter a while.

The invention claimed is:
 1. A method, comprising: periodicallytransmitting, by a sensor controller, an uplink signal; determining, bya stylus, upon receiving the uplink signal, a transmission and receptionschedule for a downlink signal and the uplink signal on a basis of areception timing of the uplink signal; executing, by the stylus, anuplink signal receiving operation for a first predetermined period oftime according to the transmission and reception schedule; andresponsive to the uplink signal not received through the receivingoperation and expiration of the first predetermined period of time,starting, by the stylus, a next receiving operation to receive theuplink signal prior to a timing of starting the uplink signal receivingoperation determined according to the transmission and receptionschedule.
 2. The method according to claim 1, comprising: responsive tothe uplink signal not received through the receiving operation andexpiration of the first predetermined period of time, continuing, by thestylus, the uplink signal receiving operation for a second predeterminedperiod of time.
 3. The method according to claim 1, comprising:responsive to the uplink signal not received through the receivingoperation and expiration of the second predetermined period of time,transmitting, by the stylus, a signal different from the downlinksignal, wherein the downlink signal is transmitted in response toreception of the uplink signal.
 4. The method according to claim 3,wherein the signal different from the downlink signal is a tone signal.5. The method according to claim 4, comprising: after executing theuplink signal receiving operation for the second predetermined period oftime, repeatedly executing, by the stylus, an operation of transmittingthe tone signal for a third predetermined period of time, and executing,by the stylus, the uplink signal receiving operation for a fourthpredetermined period of time, wherein a total period of time of thesecond to fourth predetermined periods of time is equal to atransmission cycle of the uplink signal indicated by the transmissionand reception schedule.
 6. The method according to claim 4, comprising:after executing the uplink signal receiving operation for the secondpredetermined period of time, repeatedly executing, by the stylus, anoperation of transmitting the tone signal for a third predeterminedperiod of time, and executing, by the stylus, the uplink signalreceiving operation for a fourth predetermined period of time, wherein atotal period of time of the second to fourth predetermined periods oftime is not equal to a transmission cycle of the uplink signal indicatedby the transmission and reception schedule.
 7. The method according toclaim 4, wherein the stylus includes a pressure sensor that acquires awriting pressure value representing a pressure applied to a pen point,and the transmitting of the tone signal includes, in a case where thewriting pressure value indicates that the pen point is not in contactwith a touch surface, executing, by the stylus, the uplink signalreceiving operation, instead of a transmitting operation of the tonesignal.
 8. A method, comprising: periodically transmitting, by a sensorcontroller, an uplink signal; determining, by a stylus, upon receivingthe uplink signal, a transmission and reception schedule for a downlinksignal and the uplink signal on a basis of a reception timing of theuplink signal; starting, by the stylus, an uplink signal receivingoperation according to the transmission and reception schedule; andresponsive to the uplink signal not received through the receivingoperation and expiration of the first predetermined period of time,transmitting, by the stylus, a signal different from the downlinksignal, wherein the downlink signal is transmitted in response toreception of the uplink signal.
 9. The method according to claim 8,wherein the signal differs from the downlink signal is at least one offrequency, phase, or modulation.
 10. The method according to claim 8,wherein the signal different from the downlink signal is a tone signal.11. The method according to claim 10, wherein the stylus includes apressure sensor that acquires a writing pressure value representing apressure applied to a pen point, and the transmitting of the tone signalincludes, in a case where the writing pressure value indicates that thepen point is not in contact with a touch surface, executing, by thestylus, the uplink signal receiving operation, instead of a transmittingoperation of the tone signal.
 12. The method according to claim 10,comprising: deriving, by the sensor controller, a position of the styluson a basis of the tone signal and outputting, by the sensor controllerto a host processor, the position and a writing pressure value obtainedprior to reception of the tone signal.
 13. The method according to claim10, comprising: responsive to the uplink signal not received through thereceiving operation and expiration of the first predetermined period oftime, continuing, by the stylus, the receiving operation; and responsiveto the uplink signal not received and expiration of a secondpredetermined period of time, starting, by the stylus, the transmittingof the tone signal.
 14. The method according to claim 13, comprising:after executing the uplink signal receiving operation for the secondpredetermined period of time, repeatedly executing, by the stylus, anoperation of transmitting the tone signal for a third predeterminedperiod of time, and executing, by the stylus, the uplink signalreceiving operation for a fourth predetermined time, wherein a totalperiod of time of the second to fourth predetermined periods of time isequal to a transmission cycle of the uplink signal indicated by thetransmission and reception schedule.
 15. The method according to claim14, comprising: starting, by the stylus, the uplink signal receivingoperation for the fourth predetermined period of time prior to a timingof starting the uplink signal receiving operation determined accordingto the transmission and reception schedule.
 16. The method according toclaim 13, comprising: after executing the uplink signal receivingoperation for the second predetermined period of time, repeatedlyexecuting, by the stylus, an operation of transmitting the tone signalfor a third predetermined period of time, and executing, by the stylus,the uplink signal receiving operation for a fourth predetermined periodof time, wherein a total period of time of the second to fourthpredetermined periods of time is not equal to a transmission cycle ofthe uplink signal indicated by the transmission and reception schedule.17. The method according to claim 8, wherein the sensor controllerincludes a counter that indicates an elapsed time from reception of thelast downlink signal, and is configured to cancel pairing with thestylus when a value of the counter exceeds a predetermined period oftime, and the method comprises: determining, by the sensor controller,whether or not content of the downlink signal received from the stylusis consistent with a request issued through the uplink signal; andresponsive to determining that the content of the downlink signalreceived from the stylus is inconsistent with the request issued throughthe uplink signal, resetting, by the sensor controller, the counter. 18.A stylus that, upon receiving an uplink signal from a sensor controllerconfigured to periodically transmit the uplink signal, determines atransmission and reception schedule for a downlink signal and the uplinksignal on a basis of a reception timing of the uplink signal, the stylusbeing configured to: start an uplink signal receiving operationaccording to the transmission and reception schedule; and responsive tothe uplink signal not received through the receiving operation andexpiration of the first predetermined period of time, transmit a signaldifferent from the downlink signal, wherein the downlink signal istransmitted in response to reception of the uplink signal.
 19. Thestylus according to claim 18, wherein the signal different from thedownlink signal is a tone signal.
 20. The stylus according to claim 18,wherein the signal differs from the downlink signal in at least one offrequency, phase, or modulation.